Extraction of a desired compound from a source is commonly performed by solvent extraction methods.
Solvent extraction is used today on a number of starting materials, including biomass, to extract desired components. A good example of this is the extraction of taxanes from biomass.
Taxanes are a group of diterpenoid compounds, some of which have been demonstrated to be useful in the treatment of cancer and other serious diseases, such as multiple sclerosis and kidney disease. In particular, the taxane compounds paclitaxel, docetaxel, Baccatin III, 10-O-deacetylbaccatin III (10-DAB or DAB), 13-acetyl-9-dihydrobaccatin III (DHB), cephalomannine, and prostratin have been identified as useful in pharmaceutical applications. For instance, paclitaxel, is currently being used in cancer treatment (marketed as TAXOL® by Bristol-Myers Squibb). Certain taxanes, such as paclitaxel and docetaxel, can be used directly in pharmaceutical applications, without additional chemical modification, while other taxanes (DAB and DHB, for example) are viewed as precursors for the production of other taxanes such as paclitaxel and docetaxel.
The major sources of taxanes are the bark, needles and clippings of the yew (hemlock) tree, which belongs to the genus Taxus. Unfortunately, even though taxanes are more concentrated in yew than in other species of trees, the absolute concentrations are very low. For example, it has been reported that in a typical sample, yields of only 0.01% of paclitaxel are obtained from the bark of the yew tree, and in the range of from 0.003 to 0.015 percent (dry basis) of paclitaxel from the clippings and needles (Huang et al., J. Nat. Prod., 49:665, 1986.) where the first extraction is a solvent extraction. Furthermore, the yew tree is relatively rare and grows quite slowly, raising valid concerns that reforestation and resource management cannot keep up with the demand. Although synthetic and semi-synthetic pathways for producing paclitaxel have been devised, they are extremely complex and generally too costly for commercial production. Semi-synthetic processes have also been devised for producing docetaxel.
Current methods of commercial paclitaxel and other taxane production are complex and costly. The unit operations are predominantly physical methods involving: harvesting/collection, grinding, mulching, preliminary solvent extraction and separation to get a crude taxane product. Once the crude taxane product is produced, paclitaxel may be recovered and purified in additional solvent extractions and other refining steps. In many cases, the other natural taxanes are chemically converted to additional yields of paclitaxel or to docetaxel.
Current methods for isolating other compounds from starting materials also often include an initial step of solvent extraction, which removes a large amount of impurities together with the desired compounds. As a result, one or more liquid partitioning steps to enrich the concentrations of desired compounds in the extracts are often performed, followed in some cases by several chromatography steps.
A drawback of these methods is that they require large amounts of costly and, sometimes, toxic organic solvents for the extraction and partitioning steps. Commercially, this translates to very high capital and operating costs for materials, qualified expertise, qualified technical staffing, and infrastructure.
There is therefore a need for a method of isolating compounds, which method would reduce or eliminate the requirement of large amounts of toxic and costly organic solvents.